An international body that restricts the trade of endangered plants and animals added three types of hammerhead sharks to its list of protected species last week.

The decision to list great hammerheads, smooth hammerheads and scalloped hammerheads, all of which are still legal to harvest commercially off Florida, came March 11 at the World's Wildlife Conference in Bangkok.

The conference convenes under the auspices of the 1973 Convention on International Trade in Endangered Species treaty, or CITES, which has 178 signatories, including the United States.

The CITES listing didn't bar all commercial trade of the three hammerhead species, but they now can only be traded with a special CITES permit.

The annual migration of tens of thousands of sharks, including some hammerheads, up the South Florida coast made national headlines early this month and forced the closure of beaches.

But despite the impression that migration might have left, shark populations are struggling worldwide, according to a study published in the current edition of the peer-reviewed journal Marine Policy.

Author Boris Worm of Canada's Dalhousie University estimates that 97 million sharks are killed each year, with the possible range of deaths spanning from 63 million to 273 million.

Crucially, Worm estimated that the annual mortality rate amounts to between 6.4 and 7.9 percent of all sharks in world, while sharks only have a replacement rate of 4.9 percent each year. In other words, already depleted shark populations remain in decline.

Asia, where sharks are vigorously pursued for their valuable fins, accounts for an estimated 52 percent of shark mortalities annually, according to Worm's study, "Global catches, exploitation rates and rebuilding options for sharks."

Shark harvesting is much less extensive in the United States, where quotas are set annually, and where it is illegal to fin a shark at sea.

Though they remain legal to be fished commercially in Florida and throughout the east coast, the three hammerhead species listed by CITES last week are already off limits to long line fishing.

Michael Heithaus, a Florida International University shark expert who coauthored the Marine Policy paper, said the great hammerhead is the most common of the three newly listed CITES species in the Florida Keys.

They're mainly fished recreationally, he added. But he said that because hammerheads are a very fragile fish, they usually don't survive a catch and release.

"It's a pretty good indication that when a species ends up on the CITES list, you don't want to go out and fish it," Heithaus said.